The present invention relates to an electronic timepiece which operates with high accuracy and low power consumption, such as an electronic wrist watch and an electronic clock.
The accuracy of an electronic timepiece depends on an oscillating circuit used as a source for a time-base signal. Particularly, when a quartz crystal oscillating circuit is used as a time-base signal source for an electronic watch, the accuracy of the electronic watch mainly depends on a quartz vibrator of the oscillating circuit. On the other hand, the power consumption of the electronic watch depends mainly on the power consumption of a display device for displaying the time, and the power consumption of an electric circuit consisting of a time-operating circuit including the time-base signal source for the watch and a display-driving circuit for driving the display device. Recently, this electric circuit has been realized by using a semiconductor integrated circuit (hereinafter, referred to as an IC).
The power dissipated in an IC is broadly classified either as a static power which does not depend on the operating frequency in the circuitry of the IC, or a dynamic power which is proportional to the operating frequency.
On the other hand, the accuracy of the quartz vibrator mainly depends on the rate of frequency vs. temperature change for the natural frequency thereof.
The tuning fork-type vibrator having a natural vibration frequency of 32 kHz is presently widely used as the quartz vibrator for an electronic watch. However, the frequency vs. temperature characteristic of the 32 kHz vibrator is not good. Therefore, when this kind of vibrator is used to obtain the time-base signal for a watch, the watch has an error of about 10 seconds per month. On the other hand, when an AT cut quartz vibrator of 4 MHz natural frequency is used to obtain the time-base signal for a watch, the accuracy is about 10 seconds per year. Therefore, it is clear that the accuracy of the watch will increase substantially when the vibrator of 4 MHz is used in the watch.
In the past, great efforts have been made to improve the accuracy of the electronic watch. One way of improvement is to use the AT cut quartz vibrator of 4 MHz for obtaining the time-base signal. As described above, when the watch time-base signal having a higher frequency is used, the dynamic power which is a part of the total power consumption in the watch IC will be increased. Consequently, if the vibrator frequency is merely changed from 32 kHz to 4 MHz, without any change to the IC, the dynamic power of the IC will be increased by at least a hundred times.
Actually, in the case of the hand-display type quartz wrist watch which has received practical application, the total power consumption of the watch in which the vibrator of 32 kHz is used will be about 2 .mu.W, and the total power consumption of the watch in which the vibrator of 4 MHz is used will be more than 10 .mu.W. Thus, it will be understood that the watch using the 32 kHz vibrator can be operated continuously for more than five years without change of battery, while the watch using the 4 MHz vibrator can be operated continuously for only one year or less.